Field of the Invention
The invention relates in general to a wireless receiving device, and more particularly to a decision feedback channel estimation circuit and a signal processing method of a wireless receiving device, and a channel estimation method based on decision feedback.
Description of the Related Art
Digital Terrestrial Multimedia Broadcasting (DTMB) may be categorized into a single-carrier mode or a multi-carrier mode. DTMB in the multi-carrier mode is based on transmission specifications of time-domain synchronous (TDS) orthogonal frequency division multiplexing (OFDM). One data frame of a DTMB signal includes a frame header and a frame body. The frame header includes a pseudo noise (PN) sequence that may be applied for channel estimation and signal synchronization in the time domain.
FIG. 1 shows a functional block diagram of a conventional DTMB signal receiver. A received signal received via an antenna is converted into a digital format by an analog-to-digital converter (ADC) 110, processed by an auto-gain control (ADC) circuit 120 to adjust its gain, and down-converted by a mixing circuit 130, and then undergoes channel estimation performed by a time-domain channel estimation circuit 140 in the time domain according to decision feedback information and the frame header of a data frame. The estimated channel information and the frame body of the data frame are transformed to the frequency domain by a fast Fourier transform (FFT) circuit 150. The frame body is processed by an equalizer 160 according to the channel information to reduce multipath channel interference and to generate a plurality of sets of equalized data. The equalized data is converted to binary data by a decision circuit 170, and demodulated, decoded and de-interleaved by a back-end processing circuit 190 to obtain a transmission stream. For example, the decision circuit 170 may be a hard decision circuit or a soft decision circuit. In general, the time-domain channel estimation circuit 140 usually calculates a low density parity check (LDPC) code by an iteration method based on a least mean square (LMS) algorithm. Thus, the above binary data is further transformed to the time domain by an inverse fast Fourier transform (IFFT) circuit 180 to serve as decision feedback information that the time-domain channel estimation circuit 140 requires for the subsequent iteration operation on the data frame.
An equation below represents received data Rn,k (corresponding to the kth subcarrier of the nth symbol) outputted from the FFT circuit 150:Rn,k=|Hn,k|·Xn,k|·ejen,k+Nn,k  (1)
In equation (1), Xn, k is transmitted data of the kth subcarrier of the nth symbol, Hn,k is the channel gain of the kth subcarrier of the nth symbol, and Nn, k is the additive white Gaussian noise (AWGN) of kth subcarrier of the nth symbol.
The equalizer 160 divides the received data Rn, k by the channel gain Hn, k to obtain equalized data {circumflex over (X)}n,k:
                                          X            ^                                n            ,            k                          =                                            R                              n                ,                k                                                    H                              n                ,                k                                              =                                    (                                                                                                              H                                              n                        ,                        k                                                                                                  ·                                                                                X                                              n                        ,                        k                                                                                                  ·                                      ⅇ                                          j                      ⁢                                                                                          ⁢                                              θ                                                  n                          ,                          k                                                                                                                    +                                  N                                      n                    ,                    k                                                              )                        /                          H                              n                ,                k                                                                        (        2        )            
The above channel may be a common multipath channel having a channel impulse response (CIR) as shown in FIG. 2A. However, the above channel may also be a 0-dB echo channel having a CIR as shown in FIG. 2B. The amounts of energy of the two channels are substantially equal, and may thus have similar channel responses. The channel response corresponding to the CIR in FIG. 2B contains periodical notches having the channel gain Hn, k in extremely small values, such that the noise component (Nn, k) in equation (2) is amplified, hence affecting the accuracy of the equalized data {circumflex over (X)}n,k. The number of the notches gets larger as the length of the channel gets longer, and the probability of equalized data {circumflex over (X)}n,k being affected by the noise also is increased. FIG. 3 shows a schematic diagram of a position of the equalized data {circumflex over (X)}n,k on a constellation diagram. In FIG. 3, 16 quadrature amplitude modulation (16 QAM) is taken as an example, dotted circles represent positions of constellation points, and a solid circle 310 represents the position of the equalized data {circumflex over (X)}n,k on the constellation diagram. It is known from equation (2) that, when the received data Rn, k outputted by the FFT circuit 150 is divided by the channel gain Hn, k having an extremely small value, the value of the equalized data {circumflex over (X)}n,k is increased, i.e., the position of the equalized data {circumflex over (X)}n,k on the constellation diagram is caused to get further away from the origin of the constellation diagram. Thus, the equalized data {circumflex over (X)}n,k that may originally be determined as the constellation point 320 (e.g., at the position of the solid circle 340) may be erroneously determined as a constellation point 330 by the decision circuit 170 as the equalized data {circumflex over (X)}n,k is shifted to the position of the solid circle 310. As the amount of such type of erroneous data increases, the time-domain channel estimation circuit 140 may fail to effectively perform channel estimation according to the feedback information of the IFFT circuit 180, in a way that the performance of the receiving circuit is degraded or even the accuracy of the overall received data is reduced.